Tethered platform flotation

ABSTRACT

This invention relates to a structure floating on a body of water. The structure comprises a working deck with buoyancy means supporting the deck. The buoyancy means comprises one or more slender vertical floats which have a unique structure having two parts. The first part comprises a straight, vertical, prismatic volume which runs the entire vertical length of the vertical floats. The volume of the prismatic portion comprises between about 40 and 80 percent of the total displacement. There is a second or auxiliary volume of displacement which is submerged below the trough up the maximum wave to be expected. The relative buoyancy of the two parts can be adjusted so as to minimize the vertical forces on the structure due to passing waves. This invention is concerned with additional means to vary the flotation of the prismatic part and to control the water plane area of the floats so as to reach smaller minimum variations of vertical forces on the structure.

United States Patent Silverman [151 3,654,886 [451 Apr. 11, 1972 [54]TETHERED PLATFORM FLOTATION [72] Inventor:

[73] Assignee: Amoco Production Company, Tulsa, Okla. 22 Filed: June 24,1970 [21] App]. No.: 49,417

Daniel Silverman, Tulsa, Okla.

Primary Examiner-Trygve M. Blix Attorney-Paul F. Hawley and John D.Gassett ABSTRACT This invention relates to a structure floating on abody of water. The structure comprises a working deck with buoyancymeans supporting the deck. The buoyancy means comprises one or moreslender vertical floats which have a unique structure having two parts.The first part comprises a straight, vertical, prismatic volume whichruns the entire vertical length of the vertical floats. The volume ofthe prismatic portion comprises between about 40 and 80 percent of thetotal displacement. There is a second or auxiliary volume ofdisplacement which is submerged below the trough up the maximum wave tobe expected. The relative buoyancy of the two parts can be adjusted soas to minimize the vertical forces on the structure due to passingwaves. This invention is concerned with additional means to vary theflotation of the prismatic part and to control the water plane area ofthe floats so as to reach smaller minimum variations of vertical forceson the structure.

18 Claims, 10 Drawing Figures PATENTEDAPR 11 I972 3, 654,886

SHEET 1 OF 5 INVENTOR. FIG. I DANIEL SILVERMAN TTORNEY PATENTEDAPR 1 1I972 SHEET 2 OF 5 A B C 2 2 2 e e F F F E V I E M L E E E W A S E W EIIQI O W S F 2 W M O O I I C H I 2, O F E E S T T I 0 FS .HVH OF F W 0%E W IO M I m V T m I A E M E S 2 v 0 II. T O 5 FW 0% T.- 4 F I allE IIll 58 2 W W m 5 O R I M E O O O O O O O O O O O O. O O O O O A EmE twomom E0:

mw .52 mo zoi LEADING CREST FOLLOWING CREST CREST FOLLOWING TROUGH CRESTINVENTOR. DANIEL SILVERMAN TORNEY PATENTEDAPR 1 1 I972 SHEET 3 [IF 5 MQ- MAM;

A N M FIG. 6

N R A m N E VV mu 8 ID E N A D XM A/r bvey PAIENTEIIAPR 1 1 I972 SHEET l[IF 5 92 SERVO AMPLIFIER MANMEML ACCELEROMETER 86 TENSION MEASURINGMEANS FIG.7

INVENTOR. DANIEL SILVERMAN TTORNEY PATENTEDAPR 1 1 I972 3. 654, 8 86SHEET 5 OF 5 I no H0 H2 H2 I8 38 /e/ j 46 46 M [Li /36 F o n4 L0 TOYANCHOR T Tb ANCHOR INVENTOR. DANIEL SILVERMAN A TORNE Y CROSS REFERENCETO RELATED APPLICATIONS This application is related to application Ser.No. 754,628 entitled Vertically Moored Platforms filed Aug. 28, 1968,now abandoned, and to a continuation-in-part application of thatapplication, Ser. No. 17,485, entitled Vertically Moored Platforms,filed Mar. 9, 1970, in the name of Kenneth A. Blenkarn.

BACKGROUND OF THE INVENTION 1. Field ofthe Invention This inventionrelates to a structure floating on a body of water. More particularly,the invention relates to a floating structure from which drilling orproduction operations are carried out. In general, the structure may berestrained by vertical tension members or tethers, or conventionalcatenary anchor cables, or hydraulic or hydromechanical thrustorpositioning means to have a minimum of horizontal motion. This inventionis particularly applicable to a floating structure having buoyancy meansof a specific type adapted to minimize vertical heave forces and heavemotion of the structure due to passing waves.

2. Setting of the Invention In recent years there has been considerableattention attracted to the drilling and production of wells located inwater. Wells may be drilled in the ocean floor from either fixedplatforms in relatively shallow water or from floating structures orvessels in deeper water. The most common means of anchoring fixedplatforms include the driving or otherwise anchoring of long piles inthe ocean floor. Such piles extend above the surface of the water with asupport or platform attached to the top of the piles. This works fairlywell in shallower water, but as the water gets deeper, the problems ofdesign and accompanying costs become prohibitive. In deeper water it iscommon practice to drill from a floating structure.

In recent years there has been some attention directed toward manydifferent kinds of floating structures, for the most part maintained onstation by conventional spread catenary mooring lines, or by propulsionthruster units. One scheme recently receiving attention for mooring isemployed in the so-called vertically moored platform. One such platformis described in US Pat. No. 3,154,039, issued Oct. 27, 1964. A keyfeature of the disclosure in the patent is that the floating platform isconnected to an anchor base only by elongated parallel members. Themembers there are held in tension by excess buoyancy of the platform.This feature offers a remedy for one of the major problems arising inthe conduct of drilling, or like operations from a floating structure.This major problem is that ordinary hull-type barges or vessels, inresponse to ocean waves, may exhibit substantial amounts of verticalheave and angular roll motion. Such motions significantly hinderdrilling operations. Motion difficulties are alleviated to a degree byuse of the so-called semi-submersible vessels or structures in whichflotation buoyancy is provided by long, slender vertical bottles ortanks. This design suffers the inconvenience that, if carried to thelogical extreme of having very little water plane area, the unit wouldbecome statically unstable, requiring careful reballasting to offsetchanges in vertical loads, such as drilling hook load (e.g., whenpulling drill pipe, etc.) or changes in weight of supplies. Some ofthose problems are eliminated or at least reduced in the verticallymoored platform. Being subjected to tension, the elongated parallelmembers of the vertically moored platform are substantially inextensibleand therefore restrain the platform to move primarily in the horizontaldirection. This virtually eliminates heave and roll motions. Invertically moored structures heretofore considered, exceptionally strongmooring would be required to resist the vertical forces which might beimposed upon a structure by the orbital motion of passing waves. Meansare described to minimize the mooring forces imposed by the structure onthe elongated members, such as those caused by passing waves.

BRIEF DESCRIPTION OF THE INVENTION This invention is preferably appliedto a floating structure having limited lateral movement for use in abody of water,

5 which is especially designed for an expected maximum wave.

This expected wave is usually called the maximum design wave. Thestructure includes a working platform supported by a buoyancy meanscomprising a plurality of slender vertical float members. The floatmembers are rigidly anchored to the ocean floor by a plurality ofhorizontally spaced-apart, parallel, elongated members. The volume ofthe buoyancy means can be defined as comprising two parts, the firstpart resulting from a straight, vertical, prismatic shape which runs theentire vertical length of each vertical float member. The volume of theprismatic portion comprises from about 40 percent to about percent ofthe total displacement of the buoyancy means below the still water"line. The ratio of the displacement of the prismatic portion to thetotal displacement is called the prismatic ratio p. A second volume ofdisplacement surrounds the prismatic portion and comprises the remainderof the total displacement. This second volume is placed below the troughof the design wave. This critical placement of the second or auxiliaryvolume and the critical size minimizes the critical mooring forcesimposed on the vertical elongated members by the structure due to theorbital motion of the passing waves.

As will be shown later, this type of structure can be designed tocompensate the flotation and orbital (or inertial) effects so that theyessentially neutralize each other, leaving a zero resultant verticalwave effect on the structure. Since wave motion may vary over wideranges in period and height, the best that can be done in the design ofa structure is to maintain a minimum vertical force on the structure fora range of wave height and wave period. This is all taught in the abovementioned applications, Ser. Nos. 754,628 and 17,485.

This invention is concerned with means which are made part of theflotation system of the structure, by means of which the flotationsystem can be adjusted to have a different prismatic ratio p so as tobalance the buoyancy and inertial effects of the waves on the structure.

This invention is applicable to many types of floating structures, andcan be applied to any structure, whether it has been designed tominimize the buoyancy and inertial effects of the waves or not. However,this invention is most applicable to, and is most effective with, thosestructures in which a partial balance is reached between the buoyancyand inertial forces of the waves, and will be described in terms of suchstructures. More particularly, for convenience, it will be describedprimarily in terms of a vertically moored or tethered structure, andreference will be made to the above mentioned applications forstructural detail.

BRIEF DESCRIPTION OF THE DRAWINGS Various objects and a betterunderstanding of the invention can be had from the following descriptiontaken in conjunction with the appended drawings.

FIG. 1 is a floating structure typical of the type on which thisinvention can be used.

FIGS. 2A, 2B and 2C illustrate the variation in mooring force for threefundamental types of vertically moored plat forms, which consistrespectively of only one slender, vertical float member; a float membercompletely submerged, and a member combining elements of both types.

FIG. 3 illustrates one embodiment of this invention utilizing a singlecontrol buoyancy pipe.

FIGS. 4 and 5 illustrate other embodiments utilizing a plurality ofcontrol buoyancy pipes.

FIG. 6 illustrates the detail of a preferred embodiment.

FIG. 7 illustrates a complete control system utilizing the embodiment ofFIG. 6.

FIG. 8 illustrates an embodiment utilizing control buoyancy pipes on afloating structure anchored by conventional laterally set anchors andaccompanying lines.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings in whichidentical numbers are employed to identify identical parts andparticularly to FIG. 1. Numeral l designates, generally, the floatingstructure or platform. The floating structure 10 includes a deck portion12 which may have a derrick 14 mounted thereon. The deck 12 ispreferably an enclosed space where quarters, workshop area, etc., arelocated. This is to aid in streamlining the system. Various auxiliarymeans, including a port for helicopter, etc., may be provided.

The deck 12 is supported by at least three vertical float means,generally designated by the numeral 16. This includes an upper skinnyportion 18 and a lower fat portion 20. There are enough of thesevertical support means 16 to provide stability. This would ordinarily bethree or more.

The platform is anchored by suitable means to the ocean floor. Shown inthe drawing is a base plate 22. Anchor piles 24 extend into the bottomof the ocean for whatever depth is needed to secure the properanchorage, e.g., 500 feet. These anchor members are secured in place,for example, by cement 26. Connecting anchor members 24 to the workingstructure or platform are a plurality of elongated members 28alternately called risers. These elongated members 28 are preferablylarge diameter steel pipe, e.g., 20 to 30 inches in diameter. Theseelongated members 28 could be cables of wire, chain, and the like.However, it is preferred that they be pipe so that operations can beconducted from the floating structure down through them to undergroundformations. Preferably, it is desired to drill down through these pipes.

The structure shown in FIG. 1 is essentially rigid in the verticaldirection, but is relatively free to move in the horizontal direction.Restraint against horizontal movement is only the horizontal componentof riser tension, that component being proportional to the angulardeparture of the riser from true vertical. Under the action of wind,current and other steady forces, the platform will be shiftedhorizontally until the resultant horizontal restraint equals suchapplied loads. In response to wave action the platform will oscillateback and forth about the shifted or average position. The platform will,for storm wave situations, generally oscillate horizontally so as tomove with the surrounding fluid. The horizontal motion of the platformwill basically satisfy the following relation.

X the single amplitude horizontal motion of the platform.

A the horizontal, single amplitude wave motion of water at the elevationof the platform center of buoyancy. See Equation (2).

B the buoyancy or displacement of the platform.

H the hydrodynamic mass of water associated with acceleration of theplatform. For most configurations H is essentially equal to buoyancy.

M the actual weight of the platform.

T= the wave period.

T, the natural sway period, calculated from Equation (3). Water motion Ais calculated, for simple wave theories, according to the followingequation.

A =5 -21rs/x in which h wave height, crest to trough. S the submergenceof the platform center of buoyancy below still water level. )t= wavelength 5.121", by Airy Theory) Natural sway period of the platform isexpressed as T,, =L (H+M)/BM (3) in which L the length of verticalmooring lines or risers, and other symbols are as previously defined.

For most platform configurations of interest, a design wave 100 feethigh would cause the platform to move 50 feet either side of the averageshifted position. It is generally to be preferred that steady stormshift of the platform by approximately equal to the single amplitude ofthe wave induced motion. For the case just described, an appropriatedesign shift would be 50 feet. For water depth requiring vertical risers1,000 feet long, such a horizontal shift would correspond to ahorizontal restraint equal to one-twentieth of the tension in thevertical mooring lines or risers. Thus, tension in the risers shouldgenerally be between 15 and 25 times the steady horizontal storm loads.Typically required total tensions in the order of 10,000,000 pounds areto be expected. Typically such a tension could be carried by 16 or 20pipe risers which have 20 inches outside diameter with a wall thicknessof 0.625 inches.

The vertical members 16 of the structure are connected by cross bracing34. This cross bracing is preferably all located below the still waterline indicated by line 36. As mentioned earlier, this structure will besubjected to various wave forces. In Naval engineering, when designingfloating structures, or other marine structures for that matter, it isquite common to select what is known as a maximum design wave. Themaximum design wave will have a crest 38 and a trough 40.

There are concepts disclosed herein which teach the means by which themooring forces are minimized when using structures as exemplified by theembodiment of FIG. 1. A particularly desirable shape for the verticallypositioned elongated floats is illustrated in FIG. 1. With reference tosuch a shape, the following applies. The volume of buoyancy ordisplacement can be conceived as being made up of two parts. The firstpart results from a straight, vertical, prismatic shape which has thediameter of upper portion 18 and runs the entire vertical length of thestructure. The volume of this prismatic portion of the structurecomprises between about 40 percent and about percent of the totaldisplacement. The second or auxiliary volume of displacement is thatpart which is the annulus volume between the prismatic volume and theouter wall of enlarged portion 20. This auxiliary volume is placed belowtrough 40 of the maximum design wave.

The auxiliary volume should be placed in a smooth and streamlinedfashion, as indicated above, as an annular space around the basicprismatic volume. The size of the auxiliary volume in the annulusportion of the bottles should be reduced to the extent of displacementprovided by the bracing 34 within the structure which is below thetrough of the design wave. The auxiliary volume in the annular spaceshould be streamlined and flared into the basic prismatic volume to theextent practical. While I have discussed a prismatic volume and anauxiliary volume, it is to be understood that these two volumes can becontinuous and that it is not necessary that they be separated intophysical compartments.

If a vertically moored platform is to be used, it is usually necessarythat variations of vertical mooring forces, which arise in reaction toforces imposed on the structure by wave action, be minimized within therange of wave lengths of importance. Wave lengths of importance varyfrom one wave area to another but many are typically in the range offrom about 500 feet to 2,000 feet. Wave action on the structure resultsin (a) a net vertical force on the structure, (b) a net couple on thestructure due to vertical forces on individual bottles, and (c) a netoverturning moment on the structure due to horizontal wave forces. Allof these forces contribute to the variation in mooring force.

The structure shown minimizes the variation in mooring force, for therange of wave lengths of importance, by permitting offsettingcontribution from each of the contributing factors: net vertical force,net couple of vertical forces and net overturning moment. If thisstructure is not designed to obtain proper distribution of forces, oneof these forces might be overpowering. For example, if vertical forceson individual bottles are eliminated or minimized, thereby eliminatingor minimizing the net vertical forces and the net couple due to verticalforce on the structure, the variation in mooring force is due entirelyto overturning moment and can be undesirably large, especially for thelonger wave lengths. On the other hand, if the buoyancy arrangement issuch that a small amount of net vertical force is admissible for allwave lengths, there is a phenomenon associated with this force, the netcoupling of vertical forces, which causes a net reduction in overturningmoments at the larger wave lengths. Therefore, a careful selection ofbuoyancy distribution can result in a minimization of mooring forcevariations over the entire range of important wave lengths.

The vertical forces on the structure are dominated by forces which fallinto two categories: namely, (a) variable buoyancy forces and (b)vertical water acceleration forces or inertial forces. While there areother contributions to the net vertical forces, they are of lesserimportance. All of these forces on the structure were calculated byelementary, commonly understood means. However, the dominant two forceswere combined for the calculations into one net force, heave, which isdiscussed below. The two categories of dominant vertical forces act inopposite direction to one another and one of the concepts of thisdiscussion is to carefully adjust the magnitudes of these forces toobtain the desired net vertical force. This is possible with my designfor certain ratios (L/H) of the length (L) of the enlarged portion tothe total design draft (H) and for certain ratios (r) of the radius R ofthe enlarged portion to the radius R of the prismatic portion for aselected draft where L, H, R and R, are defined in FIG. 1. As shownabove, the prismatic ratio p is defined as the ratio of the displacementof the prismatic portion to the total displacement.

I shall first consider the net vertical forces on the structure. Thesevarious net vertical forces can be calculated by using the followingequation.

F net change in vertical force, positive upwards.

A('r total displacement below the instantaneous water level.

M0) design displacement, or total displacement below design still waterlevel.

k wave decay factor, i.e., 21r/)\ where )t wave length (Airy Theory).

p water mass density.

3 gravitational acceleration A( y) cross-section area (varies with depthof y) y) hydrodynamic mass coefficient and varies with depth, i.e., (y)(mass of cylinder added fluid mass)/mass of cylinder H design draft y avertical coordinate measured position upwards from the base of thebuoyancy means.

1 a vertical coordinate measured position upwards from the design stillwater level to the instantaneous water surface yH). In Equation (4)terms [A(17) A(O)] give the force due to variable buoyancy, and theremaining term gives the force due to vertical water acceleration.

Consider first two very elementary types of vertically moored structuresas shown in FIGS/2A and 2B. FIG. 2A shows a buoy consisting only of onecylinder. This buoy is moored by one or more vertical tethers such thatit is not free to move vertically, but it can move horizontally orrotate. The buoy does not have an annular, or auxiliary portion; alldisplacement is from the prismatic portion. Therefore, the prismaticratio p which is defined as the ratio of the displacement of theprismatic portion to the total displacement, equals one (p l). The threecurves in FIG. 2A show the variation of net vertical force on thecylinder due to passage of a single wave from three different wavetrains. The three wave trains have periods of 10-, 14- and -seconds; thecorresponding wave lengths are 512-, 1004- and 2048-feet, respectively.In this example and all subsequent examples it is assumed that the waveheight corresponding to each wave length equals either one-tenth of thewave length of the maximum design wave height, whichever is smaller. Inthis and most of the subsequent examples, except where noted, themaximum design wave height is IOO-feet. Therefore, the correspondingwave heights for the curves in FIG. 2A are 5 l .2-, and lOO-feet,respectively. The variation of net vertical forces is expressed as apercent of total displacement. For example, a 20-second wave causes areduction in net vertical force of about 32 percent of the displacementwhen the wave trough is aligned at the axis of the cylindrical buoy.When the crest is aligned with the axis of the cylindrical buoy there isan increase in net vertical force of about 22 percent of thedisplacement. By virtue of the decrease in net vertical force at thetrough and the increase at the crest, this example demonstrates thatforces due to variable buoyancy are dominating for the high prismaticratio. As an explanation of terminology, the term leading crest is thatpart of the wave half way between the trough and the next crest. Theterm following crest is that part of the wave train at a point one-halfway between the crest and the next trough.

FIG. 2B shows similar curves for another fundamental configuration of avertically moored structure. In this case the entire displacement iscontributed by a spherical cavity at the bottom of the buoy and theportion of the structure projecting upwards from the sphere has anextremely small cross-section. Consequently, the prismatic portioncontributes essentially nothing to the total displacement, and theannular, or auxiliary, portion contributes the entire displacement. Theprismatic ratio equals 0 (p O). the curves of FIG. 2B show that themaximum variation in net vertical force is about 30 percent of thedisplacement in the long period, 20-second wave. However, in thisexample the vertically moored structure experiences an increase in netvertical force when the wave trough is aligned with the buoy, ratherthan a decrease as with the cylindrical buoy in FIG. 6A. This exampledemonstrates that for a low prismatic ratio the forces due to verticalwater acceleration are dominating.

This study shows that for a low prismatic ratio, forces due to verticalwater acceleration are dominating while for a high prismatic ratio,forces due to variation in buoyancy are dominating. Moreover, for anintermediate value of the prismatic ratio p, there exists a balancebetween variable buoyancy forces and vertical water acceleration forcessuch that the variation in net vertical force for the wave lengths ofinterest are substantially smaller than in the two fundamental casesexamined above.

Consider for example a vertically moored structure as shown in FIG. 2C.In this case the parameters describing the physical properties of thebottle are r 1.853, (L/H) 0.5 and p 0.545. In addition, the maximumdesign wave height, h is 100 feet and the draft, H, is feet, which isthe same as in the two preceding examples. The curves of FIG. 2C showthe variation of net vertical force when the bottle is subjected to thesame three waves. In this case the maximum variation in force is about 7percent and it occurs under the influence of both the 10- and 20-secondwaves, although it is an increase for the lO-second wave and a decreasefor the 20-second wave. FIG. 2C shows that for a bottle with thisspecific distribution of displacement, vertical water acceleration orinertial forces dominate for short period waves while variable buoyancyforces dominate for longer period waves. Furthermore, there is a wave(about a 16-second wave) for which there is virtually no variation innet vertical force because there is a perfect balance between thevariable buoyancy and vertical water acceleration forces.

If the prismatic ratio had been slightly greater, buoyancy forces wouldhave dominated as in FIG. 2A, consequently the maximum variation due tothe 20-second wave (a decrease in net vertical force) would have beengreater than 7 percent. If the prismatic ratio had been smaller, as inFIG. 28, vertical water acceleration forces would have dominated andconsequently the maximum variation due to the lO-second wave (anincrease in net vertical force) would have been greater than 7 percent.Therefore, for the range of waves of interest, to 20-second periodwaves, a best balance between the two influencing vertical forces isobtained for the combination ofparameters in FIG. 2C, r= 1.853, L/H= 0.5andp=0.545.

The basic configuration of the bottle is described by any two of thethree parameters. The most fundamental set is r and L/H where p is afunction of these two parameters. On the other hand, it is convenient toexpress the design of the buoyancy members in terms of p and r. It isalso recognized that the most practical selection of r and L/H may notalways provide the minimum net vertical force and therefore some meansare required to change the parameters in operation to subject thestructure to the minimum vertical net variation in force, which bringsus to the need for this present invention.

The essence of this invention lies in apparatus which can be made toautomatically control the ratio r so as to maintain a balance, or, atleast, a smaller minimum heave force, than is possible by a fixed valueof r, for a range of wave heights and periods.

One embodiment of this invention is illustrated in FIG. 3. Here, I showthe bottle 16 (as in FIG. 1) with a tubular pipe 46 fastened along theside of the prismatic portion 18. This pipe 46 is long enough so thatits top 47 extends above the level of the maximum wave crest 38, and itsbottom end is below the trough 40 of the maximum wave. The top can beopen as shown, or closed, with slightly different effects. The bottomend of pipe 46 is provided with a closure means or valve 48 operable byrod means 50, although other types of valves can be used equally well.

When the valve 48 is open, water will rise inside pipe 46 to the samelevel as outside. Thus, there is substantially no flotation effect dueto pipe 46 (other than the negligible displacement of the walls of themetal pipe). When the valve 48 is closed, then the flotation due to theentire exterior volume of the pipe becomes effective.

As shown in FIG. 4 a plurality of pipes may be used for control offlotation. These are shown as 56, 57 and 58 fastened to the outside ofthe prism 18 by means such as welding, as indicated. These can beoperated singly or together to provide a plurality of control flotationvalues (which can be expressed as values of Ar in the design parameterr). If the flotations are designed in binary ratios of say 1, 2, 4, 8,etc., a number of flotation values equal to 2" can be obtained, where nnumber of pipes 56, etc. Thus for four pipes of proper ratios ofdiameter, different values of control flotation can be obtained byproper choice of valve closings.

If the pipes 56, 57 and 58 are placed on the outside, they add to thetotal flotation. However, they can equally well be placed inside thewall 18 where they reduce the total flotation as shown by pipe 56'.Also, I show in FIG. 5 how two pipes can be mounted one inside the otherto provide three different values of flotation dependent on which, ifany, of the volumes 70, 68 are open to the sea.

I have shown in FIG. 2 how, with assumed parameters, an optimum balancein vertical forces can be made for the range of periods from 10 toseconds and from 52 to IOO-feet wave amplitude. I have shown how forthis design, the vertical forces are balanced for a 16-second l00-footwave, while for all others there is an unbalanced force remaining. Ishow also in FIG. 2C how the longer period waves dominate in thebuoyancy effect, while for shorter period waves the inertial forcespredominate. It is thus clear that if we had means such as shown in FIG.4 with a plurality of pipes, we could design the structure such that atl6-second periods the prism 18 would have less buoyancy and theadditionally required buoyancy would be provided by part of the pipes46. Then if the wave period went to longer values we would reduce theflotation to compensate and if the period went to shorter values, wewould add flotation to compensate. Theoretically if we had enoughcontrol buoyancy available we could maintain a substantially completebalance between the two opposing sets of vertical forces for all valuesof wave period in the chosen range.

In FIG. 6 I show a preferred embodiment of my invention in the controlflotation element 74. This comprises the pipe 75 with a special type ofvalve in which an annular flow space 76 is provided between an outertapered tube 75 and an inner tapered plug 80 with surface 78. The plug80 is supported by rod 82. As the rod 82 is moved up or down theconstriction at the valve is increased or decreased. The hydraulicresistance of the annular orifice serves to reduce the rate of flow ofwater into the pipe 75 as the wave crest approaches, and reduce the rateof water flow out of the tube as the wave trough approaches. Thus thewater inside the tube will rise to level, say, 84 when the crest reachesthe pipe, (to level 38) and fall to 86 when the trough reaches the pipe(to level 40'). The flotation effect is proportional to the differencein levels, 38-84 on the crest and 86-40 on the trough. Since underresistive restraint of the orifice 76 the level difference can be madeanything desired from zero to a maximum of 38-36, etc., this embodimentprovides in one pipe, the flexibility of a great number of smallerpipes, such as in FIG. 4.

In FIG. 7 I show a system by means of which the position of the rod 82can be controlled by a servo responsive to the tension in the tether orother sensor output, to control the flotation so as to bring thefluctuation in tension to a minimum. 1 show the flotation unit 16 withthe flotation control element 74 attached to its side. The plug 80 isoperated by rod 82 which carries a rack 96 in contact with a piniondriven by motor 94 which is mounted on a bracket 97 attached to pipe 74.In the tether 86 connected to the element 18 is placed a tension sensor88. This can be of the strain gauge type or other types, all of whichare represented by commodities available on the market and well known inthe art. The output of the tension sensor 88 goes by switch 89 to line90 to a servo amplifier 92 which sends control signals over line 93 tothe drive motor 94. The servo amplifier has an additional input from asensor sensitive to the level of the sea, via line 102. If the phases ofthe signals are the same, that is, if the tension increases when thewater level increases, then the structure is buoyancy controlled, andthe flotation element 74 must be adjusted for lower flotation, that is,the valve 80 must be opened more. On the other hand, if the phases ofthe two sensors are opposite, then the structure is inertial controlled,and the flotation must be increased, that is, the valve 80 must beclosed further.

No detail is deemed necessary for the servo amplifier since this art iswell known, and systems are available for purchase and text books areavailable.

In the event that the structure 16 is not vertically tethered, then itwill be necessary to have another sensor to replace the tension sensor88. One that might be suitable would be an accelerometer (or velocity ordisplacement sensor). This is indicated as 98 which, through switch 99,can be connected to the servo amplifier 92 in place of sensor 88. Hereagain the art of acceleration sensors is well known and they areavailable on the market so further description is not deemed necessary.

Most of the description has been restricted to an embodiment comprisinga vertically tethered platform, which is restrained to have essentiallyno vertical motion. However, because the wave effects can cause verylarge forces in the tether members, it becomes important to control theflotation to bring the variation in vertical forces to a minimum for anyparticular wave condition.

There is another large class of vessels similar to the abovedescribedtethered platform. These are the semi-submersible drilling vessels. Ageneralized drawing of such a vessel is shown in FIG. 8. Here a vesselmuch like that of FIG. 1 is shown. This drilling vessel is generallyanchored by means of a number of catenary cables to anchors in the seafloor. The anchor cables 116 go around sheaves 114 and 112 to winches onthe deck 120. While these cables restrict the horizontal motion, thevessel can move vertically through a fairly wide range due to the longcatenaries.

This vertical motion, or heave, of the drilling vessel is undesirablefor a number of reasons.

I. It places great stresses on the anchor cables as the vessel is movedup and down by the waves.

2. Because the drilling tools in the hole are of substantially constantlength it is very difficult to maintain a desired value of pressure ofthe bit on the rock due to the vertical motion of the deck, andexpensive devices are required to compensate for this heave in order tobe able to drill.

3. In logging, a precise measurement of depth of the sonde in the wellis required. This is difficult to determine, without providing expensivedevices to compensate for heave of the drilling vessel.

4. In drilling, a riser pipe is required, which is fastened to the wellhead at the sea floor, and must extend up to the vessel. Normally largeand expensive slip joints and seals are required to retain the drillingmud in the riser pipe as the vessel heaves.

For all of these reasons this invention is also applicable to, and wouldbe valuable if applied to semi-submersible drilling vessels and similarstructures.

While a limited number of embodiments of this invention have been shown,various modifications can be made thereto, all of which are felt to bepart of this invention, the scope of which is to be determined only bythe scope of the appended claims.

lclaim:

l. A floating structure for use in a body of water which comprises:

a deck;

buoyancy means for supporting said deck, said buoyancy means includingat least one slender vertical float member;

said at least one vertical float member of said buoyancy means having avolume defined in two parts, the first part resulting from a straight,vertical, prismatic shape which runs the entire vertical length of thebuoyancy means, the volume of the prismatic portion comprising apredetermined portion X, of the total displacement of the buoyancymeans, and an auxiliary portion having a volume of displacementcomprising (l-X) of the total displacement, said auxiliary volume beingplaced below the trough of an expected maximum wave; and

control buoyancy means comprising at least one vertical tubular meansassociated with said at least one float member, said tubular means ofsuch length that its top is above the crest of, and its bottom end isbelow the trough of, said expected maximum wave, said tubular meanshaving adjustable closure means below said trough of said maximum wave.

2. A floating structure asin claim- 1 in which said tubular means isclosed at the top.

3. A floating structure as in claim 1 in which said tubular means isopen at the top.

4. A floating structure as in claim 1 in which said tubular means isexterior to the float member with which it is as sociated.

6. A floating structure as in claim 1 in which said tubular means ispositioned within the outer contour of the float .member with which itis associated.

6. A floating structure as in claim 1 in which said control buoyancymeans comprises a plurality of tubular means.

7. A floating structure as in claim 1 in which said adjustable closuremeans is adapted to provide an adjustable resistance to flow of waterinto and out of said vertical tubular means.

8. A floating structure as in claim 7 including servo means to controlthe resistance to flow of said closure means in response to the heave ofsaid structure.

9. A floating structure as in claim 1 in which said predeterminedportion X comprises between 40 and percent of the total displacement.

10. A floating structure as in claim 1 in which said predeterminedportion X comprises between 45 and 65 percent of the total displacement.

11. A floating structure as in claim 1 in which said structure istethered by at least one substantially vertical tension member.

12. A floating structure as in claim 11 and including servo means tocontrol the buoyancy of said control buoyancy means in response to thetension in said tension member.

13. A floa ing structure as in claim 1 in which said structure isrestricted in its range of horizontal motion by means other thanvertical tethers.

14. A floating structure as in claim 13 in which said horizontal motionof said structure is restricted by catenary anchor means.

15. A floating structure as in claim 13 and including servo means tocontrol the buoyancy of said control buoyancy means in response to atleast one parameter of the vertical motion of said structure.

16. A floating structure as in claim 15 in which said parameter is thevertical acceleration of said structure.

17. A floating structure as in claim 1 in which said structure isanchored to the floor of the body of water only by parallel elongatedmembers.

18. A floating structure as in claim 1 in which said vertical tubularmeans has its upper end open.

1. A floating structure for use in a body of water which comprises: adeck; buoyancy means for supporting said deck, said buoyancy meansincluding at least one slender vertical float member; said at least onevertical float member of said buoyancy means having a volume defined intwo parts, the first part resulting from a straight, vertical, prismaticshape which runs the entire vertical length of the buoyancy means, thevolume of the prismatic portion comprising a predetermined portion X, ofthe total displacement of the buoyancy means, and an auxiliary portionhaving a volume of displacement comprising (1-X) of the totaldisplacement, said auxiliary volume being placed below the trough of anexpected maximum wave; and control buoyancy means comprising at leastone vertical tubular means associated with said at least one floatmember, said tubular means of such length that its top is above thecrest of, and its bottom end is below the trough of, said expectedmaximum wave, said tubular means having adjustable closure means belowsaid trough of said maximum wave.
 2. A floating structure as in claim 1in which said tubular means is closed at the top.
 3. A floatingstructure as in claim 1 in which said tubular means is open at the top.4. A floating structure as in claim 1 in which said tubular means isexterior to the float member with which it is associated.
 6. A floatingstructure as in claim 1 in which said tubular means is positioned withinthe outer contour of the float member with which it is associated.
 6. Afloating structure as in claim 1 in which said control buoyancy meanscomprises a plurality of tubular means.
 7. A floating structure as inclaim 1 in which said adjustable closure means is adapted to provide anadjustable resistance to flow of water into and out of said verticaltubular means.
 8. A floating structure as in claim 7 including servomeans to control the resistance to flow of said closure means inresponse to the heave of said structure.
 9. A floating structure as inclaim 1 in which said predetermined portion X comprises between 40 and80 percent of the total displacement.
 10. A floating structure as inclaim 1 in which said predetermined portion X comprises between 45 and65 percent of the total displacement.
 11. A floating structure as inclaim 1 in which said structure is tethered by at least onesubstantially vertical tension member.
 12. A floating structure as inclaim 11 and including servo means to control the buoyancy of saidcontrol buoyancy means in response to the tension in said tensionmember.
 13. A floating structure as in claim 1 in which said structureis restricted in its range of horizontal motion by means other thanvertical tethers.
 14. A floating structure as in claim 13 in which saidhorizontal motion of said structure is restricted by catenary anchormeans.
 15. A floating structure as in claim 13 and including servo meansto control the buoyancy of said control buoyancy means in response to atleast one parameter of the vertical motion of said structure.
 16. Afloating structure as in claim 15 in which said parameter is thevertical acceleration of said structure.
 17. A floating structure as inclaim 1 in which said structure is anchored to the floor of the body ofwater only by parallel elongated members.
 18. A floating structure as inclaim 1 in which said vertical tubular means has its upper end open.